In practical day-to-day wireless charging, there are scenarios where the wireless power transmitter cannot satisfy the power needs of one or more wireless power receivers that are placed on top or in the vicinity of it because of limitations of its input power source. For example, a wireless power transmitter connected to a USB 2.0 port can only draw 2.5 Watts (W) of power from the USB port. A wireless power receiver, external or integrated into an end device such as a smartphone, tablet, etc., may be positioned on top of such a wireless power transmitter to receiver power wirelessly from the wireless power transmitter to charge the end device. During charging, if the wireless power receiver starts drawing (say) 3 W of power, then a system reset will occur as the wireless power receiver's needs cannot be satisfied by the wireless power transmitter which has only 2.5 Watts (W) of power at its disposal from its input power source, the USB 2.0 port. On recovery from reset, the same system behavior repeats—the system resets repeatedly and the wireless power receiver fails to charge the end device. Battery charging of the end device will repeatedly start and stop which has an adverse effect on the reliability and life of the battery. The user experience will be unpleasant and the consumer may steer away from wireless charging as they may not recognize what is causing such an unstable behavior. Besides, there are multiple USB standards, for example, USB 1.0, USB 2.0, and USB 3.0 and each has its own maximum power rating. Depending on the type of USB port, the power rating can vary from 0.5 W, 2.5 W, 4.5 W, etc. As a result, the consumer may be confused and frustrated with varying wireless charging behavior and performance depending on the USB port type into which the wireless power transmitter is plugged in.
Wireless power transfer must be stable for a good end user experience as the stability of the power transfer directly impacts charging time, battery reliability, battery lifetime, etc. The wireless power transfer becomes unstable whenever the wireless power transmitter, by virtue of the limitations of input power source(s), is unable to satisfy the simultaneous dynamic power needs of one or more wireless power receivers placed on top or in the vicinity of the wireless power transmitter. Hence, there is a long felt but unresolved need for a wireless power charging system that overcomes such an unstable power transfer behavior.